From: Masahiro Yamada <yamada.masahiro@socionext.com>
To: dmaengine@vger.kernel.org,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
Arnd Bergmann <arnd@arndb.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Tejun Heo <tj@kernel.org>,
"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
"David S. Miller" <davem@davemloft.net>,
masahiroy@kernel.org
Subject: [Question] devm_kmalloc() for DMA ?
Date: Wed, 8 Mar 2017 19:59:48 +0900 [thread overview]
Message-ID: <CAK7LNARbD262isY7yOgQKAcbWQHV+3dsoRjtu6S+qUpPr98qSQ@mail.gmail.com> (raw)
Hi experts,
I have a question about
how to allocate DMA-safe buffer.
In my understanding, kmalloc() returns
memory with DMA safe alignment
in order to avoid cache-sharing problem when used for DMA.
The alignment is decided by ARCH_DMA_MINALIGN.
For example, on modern ARM 32bit boards, this value is typically 64.
So, memory returned by kmalloc() has
at least 64 byte alignment.
On the other hand, devm_kmalloc() does not return
enough-aligned memory.
On my board (ARM 32bit), devm_kmalloc() returns
(ARCH_DMA_MINALIGN aligned address) + 0x10.
The reason of the offset 0x10 is obvious.
struct devres {
struct devres_node node;
/* -- 3 pointers */
unsigned long long data[]; /* guarantee ull alignment */
};
Management data is located at the top of struct devres.
Then, devm_kmalloc() returns dr->data.
The "unsigned long long" guarantees
the returned memory has 0x10 alignment,
but I think this may not be enough for DMA.
I noticed this when I was seeing drivers/mtd/nand/denali.c
The code looks as follows:
denali->buf.buf = devm_kzalloc(denali->dev,
mtd->writesize + mtd->oobsize,
GFP_KERNEL);
if (!denali->buf.buf) {
ret = -ENOMEM;
goto failed_req_irq;
}
/* Is 32-bit DMA supported? */
ret = dma_set_mask(denali->dev, DMA_BIT_MASK(32));
if (ret) {
dev_err(denali->dev, "No usable DMA configuration\n");
goto failed_req_irq;
}
denali->buf.dma_buf = dma_map_single(denali->dev, denali->buf.buf,
mtd->writesize + mtd->oobsize,
DMA_BIDIRECTIONAL);
Memory buffer is allocated by devm_kzalloc(), then
passed to dma_map_single().
Could this be a potential problem in general?
Is devm_kmalloc() not recommended
for buffer that can be DMA-mapped?
Any advice is appreciated.
--
Best Regards
Masahiro Yamada
next reply other threads:[~2017-03-08 11:07 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-08 10:59 Masahiro Yamada [this message]
2017-03-08 11:15 ` [Question] devm_kmalloc() for DMA ? Robin Murphy
2017-03-08 18:06 ` Masahiro Yamada
2017-03-08 19:48 ` Lars-Peter Clausen
2017-03-08 19:59 ` Russell King - ARM Linux
2017-03-08 20:44 ` Lars-Peter Clausen
2017-03-08 21:19 ` Russell King - ARM Linux
2017-03-08 21:33 ` Lars-Peter Clausen
2017-03-09 3:25 ` Masahiro Yamada
2017-03-09 9:20 ` Russell King - ARM Linux
2017-03-09 11:19 ` Robin Murphy
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